A waveplate is also referred to as a linear phase retarder, or as a retarder plate. A waveplate introduces a phase shift between components of polarized light transmitted through the plate. It functions in an optical system to modify and control the relative phase of constituent beams.
A waveplate is a body of material in which the refractive index differs along two unique orthogonal directions. As a result, light rays travel at different velocities in the two directions. Consequently, a ray transmitted in one direction is retarded relative to a ray transmitted in the other direction. In crystals, these two transmission directions are often referred to as ordinary and extraordinary ray directions. The path difference k.lambda. between the two rays, expressed in wavelengths, is given by EQU k.lambda.=.+-.l(n.sub.e -n.sub.o)
where
n.sub.e =refractive index of the extraordinary ray, PA1 n.sub.o =refractive index of the ordinary ray, PA1 l=physical thickness of the plate, and PA1 .lambda.=wavelength of the light ray.
"k" can be considered the retardation expressed in integrals or fractions of a wavelength. The phase difference between two rays traveling through a birefringent material is 2.pi./.lambda. times the path difference. Therefore, the phase difference, called the plate retardation .delta., may be expressed as, ##EQU1## Thus, if a phase difference of .pi./2 is introduced between the ordinary and extraordinary rays, the plate is termed a quarter-wave plate. The same characterization is true for any condition expressed by (2.pi.)m+.delta. when "m" is an integer. When "m" is zero, the term zero-order waveplate is used; when "m" is other than zero, the plate is termed a multiple order waveplate.
The simplest retardation plate is a slice cut out of a uniaxial crystal, the slice being cut so that the optic axis lies in a plane parallel to the face of the plate. Heretofore, the principal materials used in waveplate production were crystalline materials such as quartz, calcite and mica. These crystalline materials are well recognized as being highly birefringent. Because of their large birefringent values, the thickness of a zeroth order waveplate would necessarily be impractically thin. For example, the thickness of such a plate would be on the order of 25 microns. Therefore, a practical waveplate, produced from such crystalline materials, must be of a higher order, that is, a multiple of 2.pi. plus the phase difference.
A recent publication by P. D. Hale and G. W. Day, "Stability of Birefringent Linear Retarders (Waveplates)", Applied Optics, 27 (24), 5146-53 (1988), discusses various types of waveplates and their features. In particular, the publication discusses how retardance in the various types varies with temperature, angle of light ray incidence and wavelength. For example, the effect of a slight deviation in angle of incidence is magnified by the multiple order of retardation inherent in an integral, crystalline waveplate. The term "integral" indicates a unitary, crystalline waveplate composed of a single material.
The authors conclude that, for a waveplate application requiring high stability, a low order, and ideally zero order, waveplate should be chosen. Since a zero-order, integral plate is impractically thin, it is common practice to resort to compound waveplates. Thus, to obtain a 90.degree. retardation (quarter-wave), a positive plate of 360.degree.+90.degree. is sealed to a negative plate of 360.degree.. This provides the desired 90.degree. retardation required with the multiple orders cancelling out.
The related Borrelli-Seward application discloses a birefringent glass waveplate in which non-absorbing, oriented and aligned particles having a high aspect ratio are dispersed in the glass. The particles derive their birefringence through form birefringence. The glass is a phase-separable glass selected from the group consisting of lead borate and bivalent metal oxide silicate glasses and alkali metal oxide aluminosilicate glasses from which silver halide crystals are separated.
These prior waveplates are highly advantageous. However, it would be desirable to provide an increase in the degree of phase shift per unit thickness. The present invention accomplishes this desired end by employing a glass having a different system of dispersed particles.